Multi-tube burner



Nov. 28, 1967 P. TILLMANN Er AL 3,355,253-

MULTI -TUBE BURNER Filed April 10, 1964 Fig. 3.

Fig. l.

INVENTORS Gerhard Hitzemonn Achim Kulling Peter Tillmdnn (deceased) Murgoreflillmonm sole hair AGENT United States Patent 6 Claims. (Cl. 23-277 The present invention relates in general to the production of pigmentary titanium dioxide and more particularly to an improved multi-tube burner for producing pigmentary titanium dioxide by vapor phase reaction of gaseous titanium tetrachloride and oxygen in the pres ence of an auxiliary flame.

In the manufacture of titanium dioxide pigments by combustion of gaseous titanium tetrachloride with oxygen or gases containing oxygen in a reaction chamber in the presence of an auxiliary flame for maintaining combustion, wherein a burner is used consisting of several co-axially arranged tubes, the tinting strength and rutile content of the product thus obtained are unsatisfactory unless special measures are taken.

It has been suggested, earlier, that the rutile content and the tinting strength of the pigment will be optimum when the thickness of the gas stream, in the combustion of a mixture of titanium tetrachloride and oxygen, is no greater than mm. This means, however, that in large scale production the titanium tetrachloride-containing gas stream would have to be subdivided into a plurality of streams none of which are greater than 10 mm. in thickness, thus necessitating complicated burner types attended by construction and operational difliculties to a very high degree. Furthermore, attempts have been made to reach this goal by imparting a spiral movement to at least one of the reaction components but this technique also necessitates a complicated burner construction. It is also possible to produce titanium dioxide pigment of high tinting strength by using a considerable excess of oxygen but this procedure is costly and entails disadvantages in the reuse of the combustion gases for chlorination.

In addition to the disadvantages enumerated above all of the prior art burners have a common weakness in that in continuous operation hard deposits of titanium dioxide are formed on the burner which initially disturb the flow characteristics of the flame and thus lead to impairment of pigment quality; and eventually actually close-01f the burner orifices to such an extent that combustion of the gases is terminated.

These deposits of titanium dioxide are of two types and may be differentiated on the basis of their origin; first, freshly formed titanium dioxide which is. distributed throughout the entire reaction chamber by the turbulent gas flow and which is deposited on all walls as well as on the burner tube. A certain solidification of this titanium dioxide deposit, which is at first rather loose, will take place due to the heat of the flame jet. The solidified deposits of titanium dioxide may take the form of long spikes that extend partly into the flame; or may take the form of a tube-like envelope surrounding the flame. Moreover if these spike-like deposits extend into the decomposition zone of the titanium tetrachloride and oxygen, then their subsequent growth takes place directly out of the flame and this produces particularly hard deposits. Secondly, titanium dioxide deposits which form on the burner mouth caused by back turbulence of hot 3,355,253 Patented Nov. 28, 1967 gas fractions within the reaction chamber with consequent narrowing of the burner openings.

An object therefore of the present invention is to provide an improved method and means for producing high quality titanium dioxide pigmentary material, by a vapor phase reaction of titanium tetrachloride and oxygen, economically and on a commercial scale for relatively long periods of operation.

A further object of the invention is to provide an improved multi-tube burner for the production of fine particle size titanium dioxide by combustion of gaseous titanium tetrachloride with oxygen or gases containing oxygen in a reaction chamber in the presence of an auxiliary flame for maintaining combustion wherein said burner is not limited to a gas stream of prescribed thickness, is of simple and inexpensive construction and can be operated continuously without becoming plugged by deposits of titanium dioxide or other causes of malfunction.

Other objects, features and advantages of the invention will be revealed in the description of the invention which follows in which:

FIG. 1 is a diagrammatic View of a burner-reactor asscmbly;

FIG. 2 is a diagrammatic showing, in vertical longitudinal section, of one form of the improved multitube burner of this invention;

FIG. 3 is a diagrammatic showing, in vertical longitudinal section of a modification of the burner construction shown in FIG. 2; and

FIG. 4 is a fragmentary view, in vertical longituidinal section, showing a modification of the burner cap of FIG. 3.

Referring to the drawings, the improved multi-tube burner of this invention is identified generally by the numeral 10 and is adapted to be mounted on the upper end of the reaction chamber 11 as shown in FIG, 1, for producing titanium dioxide by a vapor phase reaction of titanium tetrachloride and oxygen. The burner 10 comprises three axially arranged tubes 12, 13 and 14 respectively, each of a different overall diameter and in the present embodiment arranged substantially concentrically with respect to each other. For purposes of identification tube 12 will be hereafter referred to as the inner tube, tube 13 as the intermediate tube and tube 14 as the outer tube. Each tube is made of a suitable material to withstand relatively high temperatures and to this end may be made of a heat resistant steel, a nickel alloy or possibly silica or other suitable ceramic materials. The diameters of the respective tubes are also selected so as to provide an annular gas passage 15 between the inner tube 12 and the intermediate tube 13; and an annular gas passage 16 between the intermediate tube 13 and the outer tube 14. The bore 17 of the inner tube 12 constitutes a third gas passage. The upper ends (not shown) of the tubes 12, 13 and 14 of the burner 10 are adapted to be connected to suitable gas sources, here after described, which feed the respective gases through .the gas passages in separate streams.

Referring again to FIG. 2, the open ends of the inner tube 12, and the intermediate tube 13 are substantially flush whereas the corresponding end of the outer tube 14 is extended beyond the open ends of the tubes 12 and 13. It has been discovered that the extension of the open end of the outer tube 14 beyond the open ends of the tubes 12 and 13 is a vital factor in the effective operation-of the burner. If this extension, which is indicated at 140, is too short the burner flame will flicker and will be easily extinguished Whereas if the extension is too long the re action between the gases will be initiated before the gases have passed through the extension of the outer tube 14.

3; As aconsequence. titanium. dioxide deposits willbe formed on the burner tubes and quickly plug up the burner. The length of the extension 140-depends in general upon the capacity of the burner and. in. this respect may. vary. from 5 to 100 mm. depending on gas. flow rates, the size of burner tubes, and similar factors,

TihC extremity of the extension 140v is. provided. with a restricted aperture 18.which. serves tov modify the flow. of gases issuing from: the gas passages. of the burner. More particularly it has-been found thatthe cross-sectional; area of the aperture 18 should not exceed the free cross-sectional. area, of the burner. The phrase. freecross-sectional area. of theburner. will be understood. to mean. the sum of the cross-sectionalt areas. of the, gas passages, 15, 16 and. 17.. i

In general. the optimumsize of the aperture 18 will depend upon the throughput of titanium tetrachloride. A larger throughput requires a larger aperture and a. lesser throughput requires a smaller aperture for a given burner. The minimum size of the aperture 18; is. determined by such considerations as the particle size of the titanium dioxide produced and the operation of the burner. In gen: eral. the particle size of the titaniumv dioxide decreases as the. size of the aperture. 18: is decreased. Moreover, if the aperture 18 is too small the flame will be unstable and readily extinguished. It is. not surprisingtherefore that by calibrating the size of the restricted aperture 18. of the extension 14.0 a standardization of the tinting strength of the pigment is possible. In. this. connection: high tinting strengths are assured provided the cross-sectional area of the restricted aperture 18. does not, exceed. about 0.85- of the free. cross-sectional areaof the burner.

Thus while the length of the extension 14.0. of the outer tube- 14 is an important. factor in the operation ofthe burner continuously and. without plugging over extended periods of time, the quality of the pigment produceddepends, in the main,. upon the cross-sectional. area. ofthe restricted aperture 18 in the extension. 140. as related to the free. cross-sectional area of the burner.

Inv thebur ner shown in FIG. 2 the extension 140, is shown as an integral part of theouter tube 14. andisprovided with an annular inwardly projecting, lip. 19 which is. in a plane substantially at right. angles to the wall of the tube 14 and defines the restricted, aperture 18.

The invention is not limitedhowever to this. construction and- FIG. 3 showsa-modification wherein the functions of the extension 140 are incorporated in a cap-like member 21 which may be constructed as. an integral part of the outer tube 14 but is preferably designed, as a removable cap. Thus by providing a plurality o cap-like members 21 of different dimensions, i.e. having different lengths and aperture openings, the capacityof a given burner and/or the quality of the pigment produced may be varied by simply substitutingdifierent caps.

Althoughthe cap/21 is shownwith a sloping innerwall 22 which blends into the annular Wall of the restricted aperture 18 the particular shape of. the inner wall of the cap i,e. whether sloping or at right. angles to the plane of the aperture 18. is not significant and either construetionwillserve to modify thev flow of gasesissuing from the gas passages of the burner. It is also Within the purview of the instant invention to design the cap 21 with a double wall 23 (see FIG. 4-) so as to form an annular duct 24,within said, cap with means connectedthereto for circulating air, water or other coolants through the cap duct thereby reducing the effect of heat radiation at the apertured end or mouth of the cap. In this connection the invention also contemplates a double-walled cap having aduct 24. the inner wall 25 of which is formed of a porous material i.e. gas permeable metal or ceramic material such that an inert gas may be. bled through the pores of the inner wall of the cap to prevent the formation and/ or accumulation of loose deposits of titanium dioxide thereon.

As mentioned above whereas the burners of theprior art which. use a. mixture of gaseous titanium. tetrachloride and oxygen have been limited to a gas stream no greater than 10 mm. thick the burner of this invention has no such limitation and hence is adaptable to large scale pro duction. In this connection the gaseous titanium tetrachloride is fed into the inner tube 12 of the burner 10 preferably without admixture with oxygen and in lieu thereof additions of other gases as for example silicon tetrachloride and/ or aluminum trichloride may be added for effecting improvement in pigment quality. Some of the oxygen required for combustion may be admixed with the gaseous titanium tetrachloride if desired but the bulk of the oxygen required for conversion of the titanium tetrachloride to titanium dioxide is fed into the annular gas passage 15. Carbon monoxide is most commonly employed as the combustible gas for maintaining the auxiliary flame in the reaction chamber, and is fed into the annular gas passage 16 of the burner. Itwill be understood that other gases suchas hydrogen, hydrogen containing gases, hydrocarbons or mixtures of these gases may be used also in lieu of carbon monoxide forsup porting the auxiliary flame.

As will be seen from the drawings the burner accordingtto this invention is of: relatively simple and economical construction and although the respective feed tubes 12', 13. and 14' are shown insubstantially concentric relationship an exact concentricity is not mandatory; nor is it necessary to obtain equalization of the circumferential gas velocities. of the oxygen and carbon monoxide.

In operation, the flame produced by the burner 10 is formed down in the reaction chamber 11: at a distance below the mouth of the burner such that the heat sensitive edges of the burner tubes are not overheated. Consequently. the burner may be operated for extended periods of. time without the edges of the burner tubes being corroded or otherwise attacked. The titanium dioxide is produced in. the flame: below the mouth of the burner and hence only inconsequential amounts of titanium dioxide contact the edges ofthe burner tubes. Such as does is usually on the edges of the outer tube (or cap) of the burner where the deposit causes no particular trouble and; in fact, automatically drops off after some growth; or. may be readily removed'by periodically rapping the end ofthe burner or lightly scraping off the deposit; Moreover deposits of titanium dioxide, such as are formed on the burners of the prior art by. back turbulence of hot gas fractions do not occur in the burner of this invention. due to. the fact that. the extension of the outer tube (or cap) of the burner causes the gases to react in. azone removed' from the ends ofthe burner tubes and in particular the inner tube 12 of the burner. At the; same time'the effect. of chlorine gas and excessive heat: on. the edges of the burner is minimized thus, reducing; corrosion of these parts of: the burner.

Thus the improvedburner. of this inventionguarantees dependable. and safe operation and the production. of pigment of excellent tinting strength and high rutile content even with, tbroughputs of gaseous titanium tetrachloride of 5;00-kg-. per hour, and. above. Further it is not necessary to usea considerable excess of oxygen, nor to limit: the thickness of the titanium tetrachloride gas stream or resort to a complex andexpensive burner design for effecting spiral movement of the titanium tetrachloride gas-stream.

The examples which follow will further illustrate the novelty and superior operation of the burner of this invention. In these examples. thetinting strength of the pigments obtained were determinedaccordingtothe following standardized test method.

TINTING STRENGTH TEST The pigment to be tested was made into a paste with a mixture of carbon black, calcium carbonate and. linseedoil; and was then compared visually with a standard paste. The standard paste was prepared with a definite amount of a standard pigment. The amount of the pigment used in the test paste was varied until the brightness of this test paste equaled that of the standard paste. The tinting strength was calculated from the amount of the pigment required for equality of brightness of the two pastes. The higher the tinting strength, the better the pigment, a good pigment being one having at least a value of 165 by this test.

In the following examples the titanium tetrachloride was, in each instance, preheated to 350 C. and the oxygen to 250 C. The carbon monoxide was used at ambient temperature.

The duration of each experiment was about 8 hours, with the exception of Examples and 7. After this operational period, the burners were still in excellent shape so that combustion could have been continued without ill effects.

Examples 14 Ineach of Examples 1 thru 4 the same burner was used. The burner consisted of three coaxially arranged cylindrical tubes, such as shown in FIG. 2, having the following dimensions:

The outer tube was adapted to have a removable cap. The distance between the cap aperture 18 and the end of the inner tube 12 of the burner was maintained constant at 20 mm. for each of the four examples. Variations in flow rates of TiCl O and CO were made, as well as the use of caps having apertures 18 of different diameters.

A summation of data obtained from these four experimental runs is recorded in Table I. It was found that an increase in thruput of titanium tetrachloride above 50 kg./hr. Example 1) or above 75 kg./hr. (Example 2) was not possible since the high gas velocity caused the flame to break off. As indicated by the unsatisfactory tinting strength data for the pigment of Example 4 the cap opening was too large. Example 3 was repeated several times for confirmation, as shown in Examples 6 and 9, and each time the data was identical.

Examples 5-7 In Examples 5-7 the same burner was employed as in Examples 1-4 and the diameter of the cap aperture was 32 mm. The only variations made were in the distances between the cap aperture 18 and the end of the inner tube 12 of the burner.

The experimental data for runs 5-7 are recorded in Table I. It was found that when this distance was re duced below 5 mm., the flame broke off and that a considerable amount of titanium dioxide was deposited on the outside tube of the burner. Moreover in Example 7 such an amount of titanium dioxide deposited on the inside of the cap that the experiment had to be discontinued after an operating period of two hours.

Examples 8-10 In Examples 8-10 the same burner was used as in the preceding examples. In these runs (Examples 9-10) the titanium tetrachloride thruput was increased; and the size of the cap aperture 18 and its distance from the end of the inner tube 12 were also varied. The results of these experimental runs are shown in Table I.

6 Examples 11 and 12 In Examples 11 and 12, a burner of the type shown in FIG. 3 was employed, the dimensions of the burner being as follows;

The experimental data obtained from these runs is recorded in Table I.

TABLE I Example No 1 2 3 4 Distance between cap aperture and end of inner tube (mm.) 20 20 20 20 Diameter of cap aperture (mm.) 20 25 82 40 Ratio of cross-section of cap aperture and free cross sectional area of burner 0.205 0.32 0. 525 0.822 TiOh kg./hr. in inner tube 50 50 100 100 0 (std. cu. m./hr.) in intermediate tube-. 11 14 20 20 C0 (std. cu. m./hr.) in outer tube 9. 5 15 15 15 Oxygen excess, percent 3 4 4 4 Rutile content, percent. 85 85 90 Tinting Strength 1, 775 1, 700 1,700 1,525

Example No 5 6 7 8 Distance between cap aperture and end of inner tube (mm.) 5 20 42 15 Diameter of cap aperture (mm.) 32 32 32 25 Ratio of cross-section of cap aperture and free cross sectional area of burner.- 0. 525 0. 525 0. 525 0. 32 TiCh kgJhr. in inner tube 100 100 100 50 O; (std. cu. m./hr.) in intermediate tube... 20 20 20 14 CO (std. cu. m./hr.) outer tube 15 15 15 15 Oxygen excess, percent 4 4 4 4 Rutile content, percent.. 85 85 Tinting Strength 1, 600 1,700 1,425 1, 700

Example No 9 10 11 12 Distance between cap aperture and end of inner tube tube (mm.) 20 25 5 30 Diameter of cap aperture (mm.) 32 40 19 51. 5 Ratio of cross-section of cap aperture and free cross sectional area of burner 0. 525 0. 822 0. 29 0.41 TiOh kg./hr. in inner tube 300 25 450 0 (std. cu. m./hr.) in intermediate tube.. 20 52 5.4 73 CO (std. cu. m./hr.) outer tube 15 30 4.5 35 Oxygen excess, percent 4 3 Rutile content, percent 85 85 Tinting Strength 1, 675 1, 675

While the foregoing description and examples illustrate multi-tubular burners comprising three concentrically arranged tubes it will be appreciated that the invention also contemplates burner modifications wherein more than three tubes are used as for example additional concentrically arranged tubes for feeding oxygen and/or CO to the burner flame; and the use of burner tubes which are non-circular in cross section as for example rectilinear or eliptical tubes.

The invention may be carried out in other specific ways than those herein set forth without departing from the spirit and essential characteristics of the invention, and the present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive and all changes coming Within the meaning and equivalency range of the appended claims are intended to be embraced therein.

In the claims:

1. A multi-tube burner for use in the manufacture of fine particle size TiO by combustion of gaseous TiCL; with oxygen, or oxygen containing gases, in a reaction chamber in the presence of an auxiliary flame for maintaining combustion, said burner comprising at least three tubular members arranged coaxially and with their walls spaced radially with respect to each other thereby providing annular gas passages between the respective tubes, the gaseous TiCl being fed through the innermost tube of the burner, and the oxygen or oxygen containing gases and the combustible gas being fed through the annular gas passages surrounding the innermost tube of the burner, the outermost tube of said burner being constructed with an extension provided with a restricted aperture, said restricted aperture being arranged concentric with said burner tubes and spaced longitudinally at least 5.0 mm. from the open ends of said gas passages, the area of said restricted aperture being no greater than the free cross-sectional area of said burner.

2. A multi-tube burner according. to claim 1 wherein the area of the restricted aperture of said extension is at most 0.85 the free cross-sectional area of said burner.

3. A multi-tube burner for use in the manufacture of fine particle size TiO by combustion of gaseous TiCl with oxygen, or oxygencontaininggases, ina reaction chamber in the presence of an auxiliary flame for maintaining combustion, said burner comprising at least three tubular members arranged coaxially and with their walls spaced radially with respect to each other thereby providing annular gas passages between the respective tubes, the gaseous TiCL, being fed' through the innermost tube of the burner, and the oxygen or oxygen containing gases and the combustible gas being fedthrough the annular gas passages surrounding the innermost tube of the burner, the outermost tube of said burner being constructed with an extension comprising a cap-member arranged to be removably attached to the open end of the said outermost tube, said cap-member being constructed with a double wall arranged to. form a duct in said cap,

and means arranged to circulate a coolant through the duct of said cap, said double w-all'ed cap having a central aperture the cross sectional area of which is no greater than the free cross sectional area of said burner, said central aperture being arranged concentric with said burner tubes and spaced from 5 to mm. longitudinally axially from the open end of said gas passages.

4. A multi-tube burner according to claim 3 wherein the inner wall of said double-walled cap comprises a gas permeable material.

5. A multi-tubeburner for use in the manufacture of fine particle size TiO by combustion 0t gaseous TiCl, with oxygen, or oxygen containing gases, in a reaction chamber in the presence of anauxiliary flame for maintaining combustion, said burner comprising at least three tubular members arranged coaxially and with their Walls spaced radially with respect to each other thereby providing annular gas passages between the respective tubes, the gaseous TiCl being fed through the annular gas passages surrounding the innermost tube of the burner, the outermost tube of said burner having an extension the inner wall of which is constructedand arranged to make an angle of more than 90 but less than with the inner wall of said outermost tube, said extension having a restricted aperture at its terminal end, said restricted aperture being arranged concentric with said gas passages and having an area no greater than the free cross-sectional area of said burner.

6. A rnulti-tube burner according to claim 5 wherein the longitudinal distance axially of said restricted aperture from the open ends of said gas passages is at least 5 mm.

References Cited UNITED STATES PATENTS 3,069,281 12/1962 Wilson 23277 X J AMES TAYMAN IR'., Primary Examiner.

MORRIS O. WOLK, Examiner. 

1. A MULTI-TUBE BURNER FOR USE IN THE MANUFACTURE OF FINE PARTICLE SIZE TIO2 BY COMBUSTION OF GASEOUS TICL4 WITH OXYGEN, OR OXYGEN CONTAINING GASES, IN A REACTION CHAMBER IN THE PRESENCE OF AN AUXILIARY FLAME FR MAINTAINING COMBUSTION, SAID BURNER COMPRISING AT LEAST THREE TUBULAR MEMBERS ARRANGED COAXIALY AND WITH THEIR WALLS SPACED RADIALLY WITH RESPECT TO EACH OTHER THEREBY PROVIDING ANNULAR GAS PASSAGE BETWEEN THE RESPECTIVE TUBES, THE GASEOUS TICL4 BEING FED THROUGH THE INNERMOST TUBE OF THE BURNER, AND THE OXYGEN OR OXYGEN CONTAINING GASES AND THE COMBUSTIBLE GAS BEING FED THROUGH THE ANNULAR GAS PASSAGES SURROUNDING THE INNERMOST TUBE OF THE BURNER, THE OUTERMOST TUBE OF SAID BURNER BEING CONSTRUCTED WITH AN EXTENSION PROVIDED WITH A RESTRICTED 